A method for diagnosing a current sensor of a fuel cell system includes calculating an estimated duty value of a hydrogen pressure control valve, corresponding to a sensing value of the current sensor, while operating the fuel cell system. The estimated duty value is compared with an actual duty value where the hydrogen pressure control valve is controlled during the operation of the fuel cell system, thereby calculating an error value between the estimated duty value and the actual duty value. The error value is compared with a critical value in a normal range, thereby determining whether a failure occurs in the current sensor. The hydrogen pressure control valve controls a pressure of hydrogen supplied to a fuel cell stack of the fuel cell system.